Protective immunity to the asexual blood stages of malaria parasites involves both cellular and antibody-mediated mechanisms. Determinants of malarial immunity may be established soon after infection with the priming of key immune regulatory and effector cells. Analysis of cell-mediated and humoral immune responses after the onset of acute blood-stage infection may help clarify the critical components necessary for development of acquired immunity in malaria. It is an intriguing possibility that the severity of Plasmodium infection is determined by the degree of activation of a particular T-helper cell subset and the subsequent production of functionally relevant cytokines. The present study represents work in progress and was designed as a pilot study to assess cytokine gene expression using semiquantitative RT-PCR analysis in the nonhuman primate (rhesus) experimentally infected with simian malaria species Plasmodium cynomolgi (n=3) and P. knowlesi (n=3). PBMC were isolated and total RNA extracted from monkeys at baseline, 3 times at 4-day intervals during acute infection with the respective Plasmodium species, and once during the convalescent phase 1 week post-treatment. Parasitemias were monitored daily and plasma collected simultaneously with the PBMC for serology. RNA is presently being stored in 70% ethanol at -70C and soon will be used for cDNA synthesis and detection of multiple mRNA cytokine transcripts by PCR.